Rivet analyses

title: TASSO_1982_I180755

$\gamma\gamma\to \rho^0\rho^0$ between 1.2 and 2.0 GeV

Experiment: TASSO (PETRA)

Inspire ID: 180755

Status: VALIDATED

Authors: - Peter Richardson

References: - Z.Phys.C 16 (1982) 13, 1982

Beams: 22 22

Beam energies: ANY

Run details: - gamma gamma to hadrons, pi0 mesons must be set stable

Measurement of the differential cross section for $\gamma\gamma\to \rho^0\rho^0$ for $1.2 \text{GeV} < W < 2.0 \text{GeV}$. The cross section is measured as a function of the centre-of-mass energy of the photonic collision using the $2\pi^+2\pi^-$ final state.

Source code:TASSO_1982_I180755.cc

```c++ // -- C++ --

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief gamma gamma -> rho0 rho0 class TASSO_1982_I180755 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(TASSO_1982_I180755);


/// @name Analysis methods
/// @{

/// Book histograms and initialise projections before the run
void init() {
  // Initialise and register projections
  declare(FinalState(), "FS");
  declare(UnstableParticles(), "UFS");
  // book histos
  if (inRange(sqrtS() / GeV, 1.2, 2.0)) {
    book(_nRho, "TMP/nrho", refData(1, 1, 1));
  }
  else {
    throw Error("Invalid CMS energy for TASSO_1982_I180755");
  }
}

void findChildren(const Particle& p, map<long, int>& nRes, int& ncount) {
  for (const Particle& child : p.children()) {
    if (child.children().empty()) {
      nRes[child.pid()] -= 1;
      --ncount;
    }
    else {
      findChildren(child, nRes, ncount);
    }
  }
}

/// Perform the per-event analysis
void analyze(const Event& event) {
  const FinalState& fs = apply<FinalState>(event, "FS");
  // find the final-state particles
  map<long, int> nCount;
  int ntotal(0);
  for (const Particle& p : fs.particles()) {
    nCount[p.pid()] += 1;
    ++ntotal;
  }
  // find any rho mesons
  Particles rho = apply<UnstableParticles>(event, "UFS").particles(Cuts::pid == 113);
  for (unsigned int ix = 0; ix < rho.size(); ++ix) {
    if (rho[ix].children().empty()) continue;
    map<long, int> nRes = nCount;
    int ncount = ntotal;
    findChildren(rho[ix], nRes, ncount);
    bool matched = false;
    for (unsigned int iy = ix + 1; iy < rho.size(); ++iy) {
      if (rho[iy].children().empty()) continue;
      map<long, int> nRes2 = nRes;
      int ncount2 = ncount;
      findChildren(rho[iy], nRes2, ncount2);
      if (ncount2 != 0) continue;
      matched = true;
      for (const auto& val : nRes2) {
        if (val.second != 0) {
          matched = false;
          break;
        }
      }
      if (matched) {
        break;
      }
    }
    if (matched) {
      _nRho->fill(sqrtS() / GeV);
      break;
    }
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  scale(_nRho, crossSection() / nanobarn / sumOfWeights());
  Estimate1DPtr tmp;
  book(tmp, 1, 1, 1);
  barchart(_nRho, tmp);
}

/// @}


/// @name Histograms
/// @{
Histo1DPtr _nRho;
/// @}

};

RIVET_DECLARE_PLUGIN(TASSO_1982_I180755);

} ```